<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="../jquery-3.1.1.js"></script>
</head>
<style>
    * {
        margin: 0;
        padding: 0;
    }

    .a {
        height: 168px;
    }

    .d {
        text-align: center;
    }

    div img {
        vertical-align: top;
    }

    .c {
        /* position: relative; */
        margin-top: 10px;
    }

    .nav {
        position: fixed;
        top: 0;
        left: 48px;
    }
</style>

<body>
    <div class="a d ">
        <img src="./top.png" alt="">
    </div>
    <div class="b d">
        <img src="./nav.png" alt="">
    </div>
    <div class="c d">
        <img src="./main.png" alt="">
    </div>

</body>
<script>
    $(document).ready(function () {
        let aa = $('.a').height()
        $(window).scroll(function () {
            let bb =  $(document).scrollTop()
            if (bb >= aa) {
                console.log(bb);
                $('.b').addClass('nav');
                $('.c').css("marginTop","110px");
            }else{
                $('.b').removeClass('nav');
                $('.c').css("marginTop","10px");
            }
        })
    })
</script>

</html>